1. Field of the Invention
This invention relates to a method for manufacturing a semiconductor substrate comprising a semiconductor layer for device formation provided on a supporting substrate and insulated from the supporting substrate.
2. Related Art
As one type of semiconductor substrates, there is a SOI (Silicon On Insulator) substrate made by providing a monocrystalline silicon layer on a silicon substrate and interposing an insulating film therebetween. As a method for manufacturing a SOI substrate of this kind, so-called smart cut methods using lamination, of the kind shown in for example Japanese Patent Application Laid-Open No. H.5-211128, have been proposed.
In this method, as shown in FIGS. 16A through 16D, a SOI substrate is manufactured in three stages (steps). In the first stage, as shown in FIG. 16A, ions obtained by ionizing, for example, hydrogen gas are accelerated with a predetermined implantation energy and implanted into a base substrate 1 made of monocrystalline silicon. In this way, a defective layer 2 is formed at a predetermined depth in the base substrate 1. Here, a layer of the base substrate 1 above the defective layer 2 constitutes a thin film layer 1a eventually to become a monocrystalline silicon layer for device formation.
In the second stage, as shown in FIG. 16B, a supporting substrate 3 made of, for example, silicon is laminated to the upper face of the base substrate 1. At this time, an insulating film 4 consisting of an oxide film has been formed on the surface (in FIG. 16B, the lower surface) of the supporting substrate 3 in advance. Then, in the third stage, as shown in FIG. 16C, by heat treatment, the thin film layer 1a is detached from the base substrate 1 along the defective layer 2.
In this way, the thin film layer 1a is laminated to the insulating film 4 on the supporting substrate 3. After that, as shown in FIG. 16D, by polishing of the detachment face being carried out, a SOI substrate 6 having a monosilicon layer 5 is obtained. With this method, it is possible to obtain a monosilicon layer 5 of high quality. Also, the base substrate 1, while being reduced in thickness, can be reused.
When this kind of SOI substrate 6 is to be used, for example, for a power device or a surface micromachine, the monosilicon layer 5 is required to have a high thickness (for example several micrometers to several tens of micrometers). However, with the related art manufacturing method described above, to make the monosilicon layer 5 sufficiently thick it is necessary to make the ion implantation deep (make the position at which the defective layer 2 is formed deep).
Consequently, the ion implantation energy is high and a large amount of ion beam current is required. Also, an expensive ion implantation apparatus is also needed. In this case, to form a monosilicon layer 5 of thickness for example 10 .mu.m, an ion implantation energy of over 1 MeV is necessary. Also, there has been the problem that there is increased contamination by impurities such as heavy metals and damage in the thin film layer 1a and hence the monosilicon layer 5 due to heavy metal knock-on to the inside of the thin film layer 1a (the monosilicon layer 5) occurring incidentally along with the use of greater acceleration energies. Furthermore, it is not possible to reuse the base substrate 1 many times.